Generally, a hydraulic-type injection molding machine has an aspect that it is difficult to precisely control a position and a speed compared to an electromotive-type injection molding machine. In other words, in the case of the hydraulic type, as it uses hydraulic oil and a hydraulic actuator, viscosity, volume, and the like of the hydraulic oil changes according to temperature, and an inertial force is generated in the hydraulic actuator. These physical behaviors directly affect the control precision and responsiveness. In the case where, as a hydraulic pump, a variable discharge type hydraulic pump capable of controlling a discharge flow rate by varying the number of revolutions of a drive motor is used, length of the hydraulic circuit from the hydraulic pump to the hydraulic actuator gets longer, thereby influence of changes in viscosity, volume, and the like of the hydraulic oil being larger. When supposing, as a hydraulic actuator, for example, a mold clamping cylinder equipped in a mold clamping device, even though feedback control is carried out for the positions, mold clamping positions (mold closing positions) or mold closing time periods largely vary among shots, causing a trouble such as breakage and damage from colliding a movable mold with a fixed mold. This problem is more serious, when a mold closing speed is increased to shorten a mold closing time period (molding cycle time) in order to increase productivity.
Meanwhile, hitherto, a control method which aims at increasing a mold closing speed while preventing molds from colliding each other is known. JP 10-119102 A (Patent literature 1) discloses a method for controlling hydraulic pressure of a mold clamping device. In the method, a discharging pipeline communicated and connected with a variable displacement pump and a draw-off pipeline communicated and connected with a tank are respectively connected to cylinder pressure chambers, which are partitioned on both sides of a piston of a mold clamping cylinder, through a direction switching valve so as to selectively communicate therewith. On the discharging pipeline a solenoid proportional switching valve for controlling a discharge flow rate of the variable displacement pump is arranged and on the draw-off pipeline a brake valve and a throttle are arranged in parallel. It is so adapted that, when a position where the molds approach each other is detected, the solenoid proportional switching valve is so switched as to switch the operation of the mold clamping cylinder from a high speed to a low speed and the brake valve is switched so as to perform brake operation. Furthermore, a traveling speed of the molds by the mold clamping cylinder is detected. In switching the operation of the mold clamping cylinder from a high speed to a low speed, at the time when the speed is decreased by a required percentage from a traveling speed of the high speed, the brake valve is switched.